Method of pretreating cellulose to facilitate its acylation



Patented May- 14, 1946 Carl J. Malm and Loring W. Blanchard, J r., Rochester, N. Y., assignors to Eastman Kodak Com pany, Rochester, N. Y.,

Jersey a corporation of New J .No Drawing. Application August'4, 1942, a

Serial No.453,5.66 I 3 Claims. (01. sea-229) l This invention relates to the manufacture of cellulose esters of fatty acids of 3-4 carbon atoms, in which a, small amount of formic acid is employed in the pretreatment ofthe cellulose.

In the making of. cellulose esters containin a substantial amount of acetyl, pretreatment of the cellulose with acetic acid is possible and,

therefore, the cellulose is successfullyiactivated Without .any serious problems arising. .I-Iowever, in the preparation of propionic r butyric acid esters of. cellulose, containing little or'no'acetyl,

the use of small proportions of catalyst-,pandthe use of. aweaker anhydride (such as butyric or propionic) in the esterification, that cellulose may be esterified in" the presence of formic acid without decomposing any of the formic acid with the consequent wasteof anhydride, 7

We have also found that the susceptibility of cellulose derivatives containing formyl groups, heretofore,- toward instability, has been due to a susceptibility towards hydrolysis resultingfrom the comparatively high water-susceptibility of the the activation of the cellulose preliminary to its esterification, presents various problems. For instance, if acetic acid is employed for activating the cellulose, its removal is necessary before the esterification occurs. As propionic and butyric acids are not satisfactory by themselves for pretreating cellulose, it is necessary that some'assistance be provided. U. S. Patent No. 2,113,301 discloses a method of activating cellulose, using propionic or butyricacid, in which water oralcohol is present in the pretreatingacid, in order to sufiicientlyactivate the cellulose so it can be esterified. This increases the amount of anhydride which is necessary and,therefore-increases the cost of the process. Y a

An object of our invention is to provide a method ofpretreating cellulose in which the only lower fattyacid groups present are substantially all propionyl or butyryl. Another object of our invention is to provide a method of making cellulose propionates and cellulose butyrates which is inexpensive but results in a product having good stability. Other objects will appear herein.

We have found that cellulose may be satisfactorily pretreated with propionic or butyric acid if but a small amount of formic. acid is present in the pretreating liquid. We have found approximately to /2 part of formic acid, per part of cellulose pretreated, gives good results, but not more than /2 part of formic acid is necessary or desirable in the operation.

Heretofore, formic acid has not been employed in esterification processes because in. the usual types of esterification,.i. e. acetylation, the formic acid has been dehydrated or decomposed by the acid anhydride used,-'either because of the high temperature of esteriflcation or the large amount of catalyst employed, or both. Also, it has been thought that formyl groups combined withcellulose were very unstable and were readily broken off from the cellulose derivative in which they were found. We have found, however, that in the making, of cellulose propionate or cellulose butyrate in which a liquid to cellulose ratio of not cellulose esters. We have found that in the case of cellulose propionates or cellulose butyrate's in which only small amounts of formyl are present that the products are not susceptibletd water and, consequently, a stable product results.'

- Thepret'reatment method, as described in this application, is especially designed for use in combinationwithan esterification reaction in which notmore than seven parts ofthe liquidesterification mass is employed for one part of cellulose, andjpreferably the proportion is approximately 4375 parts of liquid to l o'f cellulose. By'thus maintaining a low liquid to celluloseTati-oit is possible to carryout the reaction "at; relatively low temperatures and'with comparatively small amounts of catalysts. For instance, a. satisfac toryesterification, using'propionic or butyric anhydride, can be carried out using a proportion of sulfuric acid of 1-5% based on the cellulose and at an esterification'temperature, the maximum of which is, not more than 100 F. and preferably within the range of. to 'Our invention is also especially adapted for the making of cellulose esters,'substantially all of the acyl ofwhich consists of fatty acid radicals of 3-4 carbon atoms: theja'mount of acetic acid present in the esterification mass being no-more than 5% and prefer-. ably none at all. A the liquid to cellulose ratio is restricted, there would be substantially no solvent employed such as ethylene dichloride "or more than 7:1 is used, thereby making possi l chloroacetic acid, these being unnecessary in the esterification process.

-' It is preferred that the pretreatment of the cellulose in accordance with our process be car-' ried out'by first treating a portion of the cellulose and after the treatment hasoccurredfor a short time, the remainder of the cellulose is added and the mixing or stirring is continueduntil the desired activation has occurred. Sulfuric acids also employed in the pretreatment liquid and by adding the cellulose in a plurality of steps, ordinarily two, "the initial cellulose added is partially broken down so that when the second or i subsequent'portions of cellulose are added, goes more readily than where the cellulose is added acid is desirable in the pretreatment. Converse:

ly, with a higher pretreatment temperature less sulfuric acid is used.

For the preparation of high viscosity esters it is also desirable to keep the proportion of sulfuric acid in the pretreating liquid relatively low. For example, it is ordinarily desirable to use approximately .001 part of sulfuric acid or less per part of cellulose to be esterified for preparing a high viscosity ester, if a pretreatment temperature of approximately 100 F. is to be used and a time of A; to /2 hour is employed for the initial portion of the pretreatment. For lower viscosity esters, higher proportions of sulfuric acid may be employed .in the pretreatment for the same temperature. If, however, the temperature is lower, higher proportions .of sulfuric acid are permissible even for the higher viscosity esters, as pointed out above.

The pretreatment, in accordance with our invention, is ordinarily carried out in a mixer or other like vessel in which thoroughpmixing or kneading of the mass can be accomplished. The pretreating liquid, which is preferably less than two parts per part of cellulose to be activated. is placed in the mixer and the cellulose is added thereto, preferably, at first, a half portion of the cellulose to be activated. The mixing is carried out until the mass begins to mix more easily or until the cellulose exhibits physical break down or crumbliness. The remainder of the cellulose is then added and the mixing is continued until the cellulose is activated to the degree desired. Ordinarily the time for treatment of the first portionis hour at 100 'F., while after the addition of the second portion the mixing is continued for 1 hour at that temperature. However, the various conditions ofoperation, such as temperature, proportion of sulfuric acid, the type of cellulose being treated, etc., all exert an mass was mixed at 100 F. for hour. A second portion of 1 pounds of linters was then added and the mixing was continued for 1 hour at 100 F. 13 pounds of butyric anhydride were added, the mixture was cooled to 58 F. and a mixture of cc. of sulfuric acid and 20 cc. of acetic acid were added. The reaction was allowed to proceed to a maximum temperature of 85 F. After 7% hours a smooth, very viscous solution was obtained. A mixture of 2 /2 pounds of water and-10 pounds of butyric acid was then added and the mass was allowed to stand at 100 F. for suflicient time to hydrolyze the cellulose ester to the .desired degree. The cellulose butyrate thus obtained was recovered by precipitation,

washing, .anddrying. The resulting product was influence on the time necessary so that no hard and fast rule can be drawn as to the time of treatment necessary unless all the other conditions are also taken into consideration. Ordinarily the proportion of sulfuric acid which will be employed in the pretreatment, in accordance with my invention, will be from .00005 to .006 part per part of cellulose. A very satisfactory ratio f pretreating liquid to cellulose is approximately %:1. However, the proportion of liquid for pretreating the cellulose maybe anywhere from A part to 2 parts per part of cellulose. With a small proportion of pretreating liquid it is sometimes desirable to add the cellulose in more than two portions in the pretreating operation. With a higher proportion of pretreating liquid, in some cases it may be desirable to initially add the cellulose as a whole. In those cases Where a small proportion of pretreating liquid is employed, obviously a small proportion of formic acid is to be employed in the pretreating liquid, such as on the order of 1% part of formic acid per part of cellulose. The following examples illustrate our invention:

Example I 1 pounds of cotton linters were added to a mixture of .6 pound ofbutyric acid, 90 cc. of 90% formic acid, and 1-cc.-of-sulfuric acid (sp. gr.

1.84) in a Werner-Pfleiderer type mixer and the a cellulose butyrate containing about 1% combined formyl.

' Example II 1 /2 pounds of cotton linters were added to a mixture of 1 pound of formic acid /2 pound of butyric acid, and 1 cc. ofsulfuric acid in a Werner-Pfieiderer type mixer. The mixer was run for '1 hour at F. and a further 1 pound portion of cotton linters was added. The mixing'wascontinued for2 hours at 100 F. 13 pounds of butyric anhydride were added, the mixture was cooled to 55 F. and 20 cc. of sulfuric acid diluted with'20 cc. of acetic acid were then added. The reaction was allowed to proceed to a maximum temperature of 98 F. After 13 hours a smooth, homogeneous solution was obtained. The hydrolysis, precipitation, washing, and drying were :carried out as in Example I.

The method which we have described is useful for the preparation of cellulose propionates, cellulose butyrates, or cellulose propionate butyrates, either fully esterified or hydrolyzed. The resulting products exhibited good stability as shown by the failure of theester to develop any acidity upon boiling for24 hours in distilled water.

We claim:

1. A method for activating cellulose to prepare it for esterification which comprises treating the cellulose with a mixture of a fatty acid of 3-4 carbon atoms, 0.3 'to 0.5 part of formic acid per part of celluloseand a small proportion of sulfuric acid until the desired activation has occurred.

2. vAprocess of preparing cellulose esters having a high contentof fatty acid radicals of 3-4 carbon atoms which comprises mixing half the cellulose to be esterified with a pretreating liquid essentially consisting of fatty acid of 3-4 carbon atoms, 0.3 to 05 part of formic acid per part of cellulose and a small proportion of sulfuric acid at 100 F. for hour, the proportion of pretreating liquid to cellulose to be esterified being approximately %:*1, adding the remainder of the cellulose and continuing the treatment for one hour at 100 F. and subsequently inducing esterification by adding to the'mass sulfuric acid and sufficient butyric anhydride to completely acylate the cellulose, the ratio of esterification liquid to cellulose being not allowed to exceed 7:1 and the esteriflcation temperature not being allowed to exceed 100 F.

3. A method for activating cellulose to prepare it for esterification-which comprises treating the cellulosewith' a mixture of a fatty acid of 3-4 carbonatoms, 0.3 part of formic acid per part of cellulose, and a small proportion of sulfuric acid until'the desired activation has occurred.

' CARL J;

LORING W. "BLANCHARD," JR. 

